A) Field of the Invention
The present invention relates to a semiconductor device and its manufacture method, and more particularly to a semiconductor device using a ZnO-containing substrate and its manufacture method. The present invention relates also to a template substrate usable for manufacturing the semiconductor device of this type.
B) Description of the Related Art
A hydrothermal method is used as a crystal growth method for zinc oxide (ZnO) for substrates. In the ZnO crystal growth by the hydrothermal synthesis, mixture aqueous solution of lithium hydroxide (LiOH) and potassium hydroxide (KOH) is used as minerizer. Because of this, Li is mixed at a high concentration of about 1017 cm−3 to 1018 cm−3 in a ZnO substrate manufactured by the hydrothermal method. As a ZnO layer is grown on a ZnO substrate containing Li, Li in the substrate is diffused in the ZnO layer on the substrate.
As a method of growing homoepitaxially a ZnO layer on a ZnO substrate, there are a method of forming a ZnO buffer layer at a low temperature on a ZnO substrate, thereafter performing annealing at a high temperature, and then growing a ZnO layer at a predetermined temperature, and a method of directly growing a ZnO layer on a ZnO substrate without involving a buffer layer. The details of these methods are described in “H. Kato, M. Sano, K. Miyamoto and T. Yao: Jpn. J. Appl. Phys. 42, L1002-L1005, 2003” and “H. Kato, M. Sano, K. Miyamoto and T. Yao: J. Crystal Growth 265, 375-381, 2004”, the entire contents of both documents are incorporated herein by reference. A ZnO epitaxial growth temperature is desired to be high from a viewpoint of good crystallinity. However, the higher the growth temperature is, Li is more likely to diffuse.
FIG. 6 shows a depth profile of an Li concentration in a homoepitaxial ZnO layer grown directly on a ZnO substrate, measured by secondary ion mass spectrometry (SIMS). The abscissa represents a depth from the surface of the epitaxial layer of each sample, in the unit of μm, and the ordinate represents an Li concentration in the unit of cm−3. Curves A1 to A3 are profiles measured at growth temperatures of 450° C., 650° C. and 800° C., respectively. It can be seen that the higher a growth temperature is, an Li concentration in the epitaxial layer increases.
Li as an group I element operates as an acceptor when Li is substituted for a Zn site in ZnO which is compound semiconductor of a group II-VI, whereas if Li enters between lattices, Li operates as a donor. These acceptors and donors adversely affect the electrical characteristics. The characteristics of a device using a ZnO-containing semiconductor are therefore influenced considerably.
FIG. 7 is a graph showing a relation between a Li concentration and an electron mobility in an undoped ZnO layer at a room temperature. As an Li concentration increases, an electron mobility decreases. It can therefore be known that carrier scattering occurs. An Li concentration not lower than 2×1017 cm−3 in particular demonstrates insulation, indicating that carriers are compensated.
JP-A-2007-1787, the entire contents of which are incorporated herein by reference, discloses techniques of lowering an Li concentration in a substrate by absorbing Li contained in a ZnO substrate manufactured by the hydrothermal method, by using material not containing Li.